1. Field of the Invention
This invention relates to devices for controlling the air-to-fuel ratio supplied to an internal combustion engine to the optimum level at which the torque of the engine is maximized.
2. Description of the Prior Art
Internal combustion engines of the automobiles operating on the Otto cycle are supplied with a mixture of gasoline and air; the mixture is combusted in the cylinders of the engine, and the pressure resulting from the combustion is converted into a torque by a mechanism including a piston and a crankshaft. To obtain maximum torque, the air-to-fuel ratio of the mixture must be controlled to an optimum level. Although the theoretically determined air-to-fuel ratio for obtaining maximum power in an ideal cycle is about 14.6, the optimum air-to-fuel ratio at which the torque is maximized varies with the operating conditions of the engine, such as the rotational speed of the crankshaft and the temperature of the coolant water in the cooling jacket around the cylinders thereof. Thus, to obtain maximum torque under high load conditions, it is necessary to determine the operating conditions of the engine and to control the air-to-fuel ratio supplied to the engine in relationship therewith. In conventional devices, this is usually done by an open-loop control system, as will be described hereinbelow.
FIG. 1 shows the organization of a conventional air-to-fuel ratio control device described in Japanese laid-open patent application No. 60-212643. The flow of air passing an air cleaner 1 is measured by an air flowmeter 2, controlled by a throttle valve 3, and led into an intake manifold 4 to be supplied to each cylinder 5 of the engine. The exhaust gas is led out of the cylinder 5 through an exhaust manifold 6. On the other hand, the fuel, i.e., gasoline, is injected into an air inlet passage to each cylinder 5 by a fuel injection valve 7, in an amount controlled by the control device 8, and the resulting mixture of gasoline and air is supplied into the cylinder 5 to be combusted therein by being ignited by an ignition plug 9. The sensor system for detecting the operating conditions of the engine includes, in addition to the air flowmeter 2 for measuring the amount of air which is supplied to the intake manifold 4 and mixed with the fuel injected therein, a water temperature sensor 10 for detecting the temperature of the coolant water in the cooling jacket around the cylinder 5 of the engine; a crank angle sensor 11 disposed in the distributor in the case of the device shown in the figure; and an exhaust gas sensor 12 for detecting the concentration of a component (e.g. the oxygen concentration) of the exhaust gas. The crank angle sensor 11 generates a reference position pulse at each reference position of an crankshaft (spaced 180 degrees apart in the case of a 4-cylinder engine, 120 degrees in the case of a 6-cylinder engine), and a unit angle pulse each time the crankshaft is rotated a unit angle (e.g. 1 degree). Thus, the crank angle can be determined by counting the number of unit angle pulses generated after a reference position pulse. The rotational speed of the engine, on the other hand, can be determined from the frequency or the period of the unit angle pulses. Thus, the crank angle sensor 11 also functions as a rotational speed detector.
The operation of the control device of FIG. 1 is summarized schematically in FIG. 2. The control device 8, consisting, for example, of a microcomputer including a CPU, a RAM, a ROM, and an input/output interface, computes the appropriate amounts of fuel which are to be injected by the valve 7 on the basis of the signals indicating various values detected by the sensors. The sensors shown at the right hand column of FIG. 2 includes, in addition to those shown in FIG. 1 (i.e., air flowmeter 2, speed detector or the crank angle sensor 11, water temperature sensor 10, and exhaust sensor 9), a throttle total closure switch, a starter motor switch, a battery voltage sensor, etc. The computations of the injected amount of fuel may be divided into two portions. A first portion comprises the computation of the fundamental or the starting injection amount, and the second portion comprises the corrections with respect to various operating conditions, such as the corrections with respect to the battery voltage, high load, etc., and the augumentative corrections for the water temperature and for the time after starting or idling of the engine. Thus, the control device 8 calculates the amount of fuel on the basis of the signals, including the signals S1 through S4, as shown in FIG. 1, outputted by the sensors listed in the right hand column of FIG. 2, and outputs an injection signal S5, in response to which the valve 7 injects a controlled amount of fuel calculated by the control device 8.
To explain the above operation in greater detail, the injection amount of fuel Ti is computed by the control device 8 according, for example, to the following equation: EQU Ti=Tp.times.(1+Ft+KMR/100).times.x Ts, (1)
where Tp is a fundamental injection amount, and Ft, KMR, .beta., and Ts are various correction coefficients as will be explained below. The fundamental injection amount Tp is calculated, for example, by the equation: EQU Tp=K.times.Q/N, (2)
where Q and N represent the amount of air suction and the rotational speed (i.e., revolutions per minute) of the engine, respectively, K being a constant. The correction coefficient Ft corresponds to the temperature of the coolant water of the engine, and takes, for example, an increasingly greater value as the temperature falls. The coefficient KMR, a correction factor with respect to the high load, is read out of a table of values stored in a memory of the control device 8. As shown in FIG. 3, the values of KMR are stored in a tabulated form according to the fundamental injection amount Tp and the rotational speed of the engine. Further, the battery-voltage correction coefficient Ts compensates for the variation in the operating voltage of the fuel injection valve 7. The correction coefficient .beta. is determined on the basis of the exhaust gas concentration signal S4 from the exhaust gas sensor 12. Thus, by multiplying this coefficient .beta. in equation (1), the air-to-fuel ratio can be controlled to a predetermined level (e.g., in the neighborhood of the theoretical air-to-fuel ratio 14.6) by means of a feedback control. However, when the feedback control using the signal S4 is effected, the air-to-fuel ratio of the mixture is always controlled to the predetermined constant level, thereby nullifying the effects of the corrections with respect the water coolant temperature and high load. Thus, the feedback control with respect to the exhaust signal S4 is effected only when the coefficients Ft and KMR as described above are equal to zero.
The control device as described above has the following disadvantage: Although feedback control utilizing the signal from the exhaust gas sensor is partially effected, the control system is reduced to an open-loop system without any feedback loop under the high load condition, where the amount of injected fuel Ti is determined by the fundamental injection amount Tp (which in turn is determined by the quantity of air suction Q and the rotational speed N), the rotational speed N, and the battery voltage. Thus, under the high load condition where no feedback loop exists, the random variations in the characteristics of the air flowmeter 2 and the fuel injection valve 7, or the changes in the characteristics thereof with the lapse of time, are not taken into account by the control system. As a result, the air-to-fuel ratio may be deviated from the optimum ratio (i.e., the ratio at which the maximum torque is obtained: this optimum air-to-fuel ratio varies with the operating condition of the engine and is generally different from the target value used in the feedback control with respect to the exhaust gas signal; the optimum ratio generally is, for example, around 13), thereby lowering the torque of the engine and deteriorating the stability thereof. A further disadvantage of the above described control device is this: The air flowmeter 2 measures the quantity of air retained in the air inlet passage, together with that of air which are actually taken into the cylinder 5 of the engine. Thus, even when feedback control is effected, the air-to-fuel ratio is often deviated from the target value thereof.
Thus, the Japanese laid-open patent application No. 60-212643 proposes a control device comprising a means for detecting the pressure within the cylinders of the engine. In the proposed control device, the normalized maximum pressure within a cylinder of the engine (i.e., the ratio of the maximum pressure within a cylinder in the combustion stroke to the pressure therein at the time immediately preceding top dead center), for example, is used as a value for representing the torque generated by the piston in the cylinder; a feedback control is effected to maximize this representative value, e.g. the normalized maximum pressure within the cylinder of the engine. This feedback control proposed by the Japanese patent application represents an improvement over the conventional method. However, the value taken as representative of the torque, e.g., the normalized maximum pressure within the cylinder may not faithfully represent the generated torque. Thus, even by this feedback control, the air-to-fuel ratio may be deviated from the optimum value thereof.